In injection moldings of polypropylene-based resins, a so-called injection foam molding wherein foaming is conducted for the purpose of reducing weight and cost as well as of preventing warping and shrinkage of a molded article has heretofore been employed. However, since the polypropylene-based resin is crystalline and has a low melt tension, a surface of a molded article has been subject to an appearance defect called silver streaks (or swirl mark) due to a breakage of cells during foaming, and it has been difficult to increase an expansion ratio due to an occurrence of an internal void and the like. Further, a rigidity of the molded article has not been sufficient since the cells are non-uniform and large in size.
As a method of improving a foamability, a method of increasing a melt tension of a polypropylene-based resin by adding a cross-linking agent or a silane grafted thermoplastic resin has been proposed (JP-A-61-152754, JP-A-7-109372, for example). Though it is possible to obtain a foamed molded article having a high expansion ratio with the method, a viscosity at the time of melting is increased too much by the method to make it difficult to perform the injection molding, thereby deteriorating a surface property of the obtained molded article.
Also, a polypropylene having a higher melt tension as compared with ordinary linear polypropylene-based resins owing to an introduction of a long chain branch by irradiation with radioactive rays and a so-called strain hardening property which is a property of increasing a viscosity along with an increase in drawing strain of a melted substance has been offered commercially by SunAllomer, Ltd. as HMS-PP (High Melt Strength Polypropylene) (JP-A-121704). It is possible to prepare the HMS-PP having the strain hardening property by melt-kneading a polypropylene-based resin, an isoprene monomer, and a radical polymerization initiator (JP-A-9-188774). It is known that a molded article is obtainable through an injection foam molding using such HMS-PP as a base material resin (JP-A-2001-26032). However, the HMS-PP used in the injection foam molding has a melt flow rate of about 4 g/10 min. and is low in fluidity in melting, thereby causing a short shot in molding using a die cavity whose clearance having a thin portion of about 1 to 2 mm. In turn, another HMS-PP which is known to have a higher melt flow rate (30 g/10 min.) makes it difficult to obtain a foamed molded article having a high expansion ratio since a melt tension thereof is only about 0.3 cN though the HMS-PP exhibits the strain hardening property. Further, since an expensive radiological installation is used for the preparation of the above HMS-PPs, the thus-prepared HMS-PPs inevitably become expensive, and it is difficult to provide products obtained therefrom at low prices.
Also, a polypropylene-based resin which is mixed with polyethylene having a specific limiting viscosity to achieve a high melt flow rate and a high melt tension (JP-A-2003-128854) and a method of using a mixture of a polypropylene-based resin which contains a component having a specific limiting viscosity by multi-stage polymerization and has a high melt tension and a polypropylene-based resin having a high melt flow rate for injection foam molding have been proposed (JP-A-2003-268145). However, since the above polypropylene-based resins do not exhibit the remarkable strain hardening property achieved by the HMS-PP having the long chain branch, cells are broken in the case of a high expansion ratio exceeding 2 to be subject to generation of voids, thereby failing to meet the needs of high rigidity and light weight.
As a method of obtaining a foamed molded article having a satisfactory surface appearance, a method of maintaining a pressure inside a die cavity at a value capable of preventing foaming in advance is known (JP-A-9-227425, JP-A-5-269778). However, each of the foamed molded articles obtained by the methods has a low expansion ratio of less than 2, and no example of a foamed molded article having a higher expansion ratio is disclosed in the literatures. Further, JP-A-2002-192549 discloses a process of producing an injection foamed molded article having a glossy appearance and a layer of a high expansion ratio of 1.1 to 4.0 by injecting a melt resin into which carbon dioxide has been dissolved into a die cavity maintained at a pressure for preventing foaming at a flow front of the melt resin, and forming a surface layer by pressing the melt resin against a wall of the die, followed by retrieving a movable die. Though it is possible to produce foamed molded articles having an expansion ratio of 2 or more and an glossy appearance according to the above method, a relatively high pressure (e.g. 7 MPa) is required for the preliminary pressurizing of the die, so that a pressurizer capable of enduring the high pressure is required and it is necessary to bestow much care on a sealing structure of the die. Further, the increase in pressure for the die preliminary pressurizing causes adverse effects on the foaming since a gas tends to be left inside the die when the gas is withdrawn from the die for foaming and entails the necessity of bestowing much care on timing and a method of the gas withdrawal as well as a problem of lacking in stability. Also, though examples of polystyrene and polycarbonate are disclosed in the literature, no specific example of polypropylene is disclosed in the literature.
Also, in the case of combining the above surface improving methods and the polypropylene-based resin improved in foamability, it has been difficult to obtain a polypropylene-based resin foamed molded article which is capable of maintaining a fluidity at a thin portion and excellent in surface property with a high expansion ratio of 2 or more.
In addition, though methods of improving the surface property, such as coating on a surface of a molded article, pasting a surface material on the surface, and the like have heretofore been proposed, automakers, in particular, demand omission of these methods in view of environment pollution, recycle, and cost reduction.
As described in the foregoing, it has been difficult to obtain a polypropylene-based resin injection foamed molded article which is satisfactory in injection foam molding property and high in an expansion ratio and has a remarkably light weight, a glossy surface appearance, and an excellent rigidity.